The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent that it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Selective catalytic reduction (SCR) systems have been used in motor vehicles for nitrogen oxides (NOx) reduction. The SCR system includes an SCR unit (e.g., a SCR catalyst) in which an SCR process occurs. Generally, a liquid reducing agent, such as urea, is injected into the exhaust stream upstream of the SCR unit. The injected urea solution breaks down under thermal decomposition and hydrolysis to form gaseous ammonia (NH3) and carbon dioxide (CO2). The gaseous ammonia functions as the reductant to react with nitrogen oxides (NOx) of the exhaust gas in the SCR unit to form water (H2O) and nitrogen (N2). To achieve optimum SCR performance, the urea solution needs to be properly vaporized, mixed and distributed uniformly in the exhaust gas stream, before reaching the SCR unit.
Referring now to FIGS. 1 and 2, a traditional mixing device 10 of an SCR system is generally installed in an exhaust pipe 12 upstream of the SCR unit (not shown) and has a mesh structure defining a plurality of openings 14. The mixing device 10 is arranged perpendicular to the exhaust pipe 12 so that the plurality of openings 14 are parallel to a general flow direction A of the exhaust gas. Tabs 15 often extend at the downstream end of the mixing device 10 at various angles to enhance the mixing. These tabs 15 contribute to the pressure loss in the SCR system. The urea solution is injected upstream of the mixing device 10 in direction B at an angle relative to the general flow direction A of the exhaust gas. As the urea solution and the exhaust gas flow through the openings 14 and past tabs 15 of the mixing device 10, the mixing device 10 helps vaporize the urea solution and mix the urea solution with the exhaust gas.
The urea solution may pass through the openings 14 and tabs 15 without contacting the mixing device 10 and may also impinge on the pipe wall. As a result, the urea droplets may not be properly broken down into smaller sizes to be more uniformly distributed in the exhaust gas stream. Moreover, the mixing device 10 may cause a significant pressure drop across the mixing device 10, particularly at high flow rates, and impede the exhaust flow in the exhaust pipe 12.